The present invention relates generally to lighting devices that drive a light-emitting source such as an LED (light emitting diode) lamp, and an illumination fixture using the same. More particularly, the present invention relates to an LED lighting device with circuitry for detecting deterioration consistent with an end-of-life condition in an LED lamp, and adjusting a driving output signal accordingly.
In recent years, LEDs have become increasingly popular alternative light sources for illumination with respect to fluorescent lamps. In one conventional example where LED lamps are provided in place of a fluorescent lamp, an LED lamp has a shape close to that of a conventional straight tube type fluorescent lamp. The LED lamp is provided with a light source block that is configured to mount a number of LEDs on a band plate shaped mounting board, a straight tube type glass tube that contains the light source block inside, bases that cover both ends of the glass tube, and terminal pins that are projected from side surfaces of the bases and intended to supply electricity to the light source block. Such an LED lamp is detachably coupled to a lamp socket that is provided in a dedicated illumination fixture, and lights by being supplied with power (DC power) via the lamp socket from an LED lighting device mounted in the illumination fixture.
In another conventional example of such an LED lighting device, control (constant current control) is performed in which a voltage (output voltage) applied to an LED lamp (lamp socket) and a current (output current flowing through the LED lamp) are detected, and the output voltage is adjusted such that the output current becomes equal to a target value (e.g., a rated current of the LED lamp).
However, in the case where a failure such as disconnection or short circuit occurs in the LED lamp, if the LED lighting device keeps supplying power on the basis of the constant current control, the output voltage may increase to an abnormal voltage that exceeds a rated voltage of the LED lamp, or an overcurrent may flow through the LED lamp. For this reason, in the conventional LED lighting device, a control function (lamp abnormality monitoring control) is performed in which an upper limit voltage that is sufficiently higher than the rated voltage of the LED lamp, and a lower limit voltage that is sufficiently lower than the rated voltage of the LED lamp are set. If the output voltage applied to the LED lamp exceeds the upper limit voltage, or falls below the lower limit voltage, the output voltage is reduced or disabled.
That is, if in response to, for example, a deterioration over time a failure such as disconnection or short circuit occurs in the LED lamp, the LED lighting device reduces or disables the output voltage on the basis of the lamp abnormality monitoring control function, and therefore it may be possible to avoid overstressing circuit components that configure the LED lighting device.
Deterioration of an LED lamp is largely influenced by the environment in which it is used (in particular, temperature environment). For example, if an LED lamp that is often used under an environment having higher temperatures than room temperature is compared with an LED lamp that is often used under an environment having lower temperatures than room temperature, it is generally expected that the former LED lamp will deteriorate faster than the latter.
However, in the case where an LED lamp is used over a period of time exceeding a rated life for an illumination fixture (and LED lighting device), if the above-described upper limit value of the output voltage is fixed, a failure (disconnection) of the LED lamp that has accordingly slowly deteriorated may be missed.